The present invention relates to ceramic honeycombs useful as catalysts or catalyst supports for catalytic reactors, and more particularly to a method for making extruded aluminum oxide (alumina) or other ceramic honeycombs having improved properties.
Ceramic honeycomb structures composed of alumina and produced by the extrusion of plasticized alumina powder batches containing appropriate binder constituents are well known. U.S. Pat. No. 4,631,267 to Lachman et al., for example, describes a variety of permanent binder materials useful for producing relatively strong honeycombs of alumina at relatively low temperatures.
Boehmite (alumina monohydrate AlOOH ) and pseudoboehmite are known to be useful as binders for extruded honeycomb substrates formed of oxides or oxide compounds such as alumina, silica, titania, zirconia, spinel, zeolites, and the like. The binding strength of these boehmites is thought to come from strong hydrogen bonding among these and other alumina species in the extrudate and the development of an integrated Alxe2x80x94Oxe2x80x94Al network during calcination of boehmite in which it undergoes a phase transformation to xcex3-alumina.
Extrusion batches that contain boehmite as a binder for alumina or other oxides also typically include organic extrusion aids that behave as temporary binders for the mixtures. Although eventually removed during calcination, these temporary binders provide green strength through the drying process and mechanical integrity as the ceramic article is calcined to its final state.
Unfortunately, large high surface area honeycombs extruded with combinations of these permanent and temporary binders, including for example large honeycombs formed of xcex3-alumina, are frequently found to crack severely during drying. While the origin of this cracking is not completely understood, it is likely the result of heat-induced changes in the hydrogen-bonded boehmite network as well as to stresses that develop as a result of interactions between the boehmite binders and the temporary organic binders.
Another key aspect of ceramic honeycomb manufacture, particularly for catalyst support applications, is the need to maintain control over porosity of the calcined part. Porosity affects both the physical properties of the honeycomb and its suitability as a support for various catalysts. U.S. Pat. Nos. 4,001,144 and 4,868,147 provide examples of the use of various alumina precursors to provide catalyst support materials, and processes for controlling the porosities of the materials. However, much of the technology developed for producing pelletized alumina extrudates or beads for catalyst supports cannot be directly transferred to honeycomb production because of the much higher susceptibility of the thin-walled honeycombs to structural damage during the extrusion, drying and firing stages of manufacture.
In accordance with the present invention, the structural integrity of extruded ceramic honeycombs, particularly as that integrity may be adversely impacted during the drying and firing stages of the honeycomb manufacturing process, is significantly improved. These improvements are realized through the approach of developing the permanent binding phases needed for honeycomb strength in situ within the honeycomb structure after extrusion, rather than by including all of the permanent binding constituents in the initial extrusion batch. Thus, whereas in conventional manufacturing processes permanent binder ingredients such as boehmite and/or other hydrous alumina materials are directly added to the extrusion batch for mixing and extrusion with the alumina powder batch constituents, the present invention foregoes such additions in favor of a process step that causes the growth of a permanent boehmite binding network within the honeycomb, typically as a prelude to or as a part of the extrudate drying process.
While all of the mechanisms responsible for the observed improvements in honeycomb crack resistance and strength are not yet fully understood, one important aspect is thought to reside in the relatively gradual development of boehmite or pseudo-boehmite binding phases within the honeycomb wall structure. Through this gradual development it is thought that the normal stresses arising during the development of a hydrogen bonding network from such phases, which can be significant in the case of a bulk addition of boehmite, are checked or moderated.
One aspect of the invention therefore resides in an improved method for the manufacture of a high strength, substantially crack-free ceramic honeycomb structure. In accordance with that method a moldable ceramic powder extrusion batch is first compounded. The extrusion batch comprises a ceramic powder, a water vehicle and a cellulosic binder, with the ceramic powder including at least one high-surface-area boehmite precursor.
By a high-surface-area boehmite precursor is meant a finely-divided transition alumina powder that can be converted to boehmite, pseudo-boehmite or other boehmite-type (structurally similar) alumina by heating in the presence of water below 100xc2x0 C. The transition alumina can be crystalline, amorphous, or a mixture of both. Hence these precursors are largely water-free transition alumina powders, i.e., incorporating less than the approximately 30 wt % of structural and intercalated water typically found in boehmite, that will take up additional structural water and transition to a boehmite-type structure in the presence of heat and moisture. The ceramic powder component of the extrusion batch may consist entirely of one or more boehmite precursors, or it may comprise other aluminas, or other ceramic powders, to be bound into the ceramic honeycomb through the in situ development and calcination of the boehmite-type permanent binder.
The moldable ceramic powder extrusion batch thus provided is next shaped into a water-containing green honeycomb preform. Shaping is preferably carried out by extrusion, although other shaping methods including molding may alternatively be employed.
Following shaping, the water-containing green honeycomb preform is next heated under moisture-retaining conditions at a temperature and for a time at least sufficient to develop a boehmite binding phase in situ in the green honeycomb. This binder development or so-called hydration heat treatment will be of a duration sufficient to achieve the predetermined level of alumina hydration and in situ binder development within the honeycomb required to develop the degree of crack resistance and ultimate honeycomb strength desired in the final product.
In general, moisture-retaining conditions suitable for the practice of the invention are those conditions that will insure at least some water retention in the green honeycomb during this heating step. Physical means such as wraps or enclosures may be used for this purpose, or moist or humid conditions may be maintained, or moderate temperatures that retard the rate of water evaporation and drying of the structure can be used. What is required is simply that total loss of water from (complete drying of) the preform does not occur over the duration of the treatment. By a boehmite binding phase is meant boehmite, pseudo-boehmite, or other boehmite-type, or some other highly hydroxylated alumina phase evidencing a structural water loss peak at temperatures in the 300-450xc2x0 C. temperature range, as characteristic of synthetic boehmite.
Once the desired level of alumina hydration in the honeycomb has been reached, the honeycomb is dried and calcined by further heating. The particular heat treatment employed will depend on the composition of the honeycomb and degree of ceramic powder reaction and/or consolidation desired, but increased strength at any particular level of retained porosity in the calcined ceramic is readily attainable.
In another aspect the invention provides strong, high-surface-area honeycombs consisting essentially of alumina that are particularly useful as catalysts and catalyst supports for a variety of different catalyst reactions. Thus, in addition to high strength these honeycombs offer a desirable range of macro- and meso-porosity that is particularly well adapted for applications such as the catalytic processing of gas and liquid feed streams comprising convertible hydrocarbon species.